This invention relates to a process of cooling glass by introducing it into a fluidised bed of particles under thermal conditions such that heat is transferred from the glass to the fluidised material.
The invention has been made in the course of attempts to solve problems encountered in the thermal tempering of glass sheets and will hereafter be more particularly described in that context. The invention is however not restricted to a thermal tempering process.
The thermal tempering of glass involves rapid cooling of the glass from a temperature above its strain point usually a temperature near its softening point. The surface layers of the glass are cooled more quickly than the internal layers with the consequence that the surface layers become subjected to compressive stresses balanced by tensile stresses in the internal layers.
A long established cooling process employs cooling air streams which are directed against the face of the glass. Problems arise when using this process in the tempering of some thin glass sheets, e.g., sheets of ordinary soda-lime glass less than 3 mm in thickness. In particular the extraction of heat from the surface layers of glass at a rate sufficient to establish the required stress gradient within the thickness of the glass sheets necessitates impingement of cooling air against the very hot glass at a flow rate which under those thermal conditions tends to deform the thin glass sheets and/or to spoil the faces of the glass. Such defects may render the sheets unusable for their intended purpose and they certainly do so in the event that high quality optical and other specifications are demanded, as for example in the manufacture of some vehicle windows such as automobile windscreens.
In the glass industry interest has in recent years been shown in the use of fluidised beds of particulate material for effecting the rapid cooling of glass sheets which is required for tempering purposes. This interest is explained by the large heat exchange coefficients which such fluidised beds can have.
One of the factors which influences the rate of heat exchange between a hot glass sheet and the particles of a fluidised bed into which the glass sheet is immersed is the velocity of the fluidising gas stream.
The rate of heat exchange increases with increase in the gas velocity because of the higher mobility of the fluidised particles. However, the more the fluidising gas velocity is increased the more risk is there of the quality of the glass article being impaired. There are a number of factors which can give rise to this risk, one of them being differences in conditions from one zone to another in the fluidised bed, e.g. because of bubble formation. The adverse effects on the glass article may be deformation of its shape and/or impairment of its surface. The risk of deformation is particularly present when treating thin glass. When using a fluidised bed for cooling glass through its strain point and sufficiently rapidly to effect thermal tempering, any lack of uniformity of conditions within the fluidised bed is liable to lead to non-uniform tempering of the glass.